1. Field of the Invention
The present invention relates to a ferrite powder type magnetic toner used in electrophotography and a process for producing the ferrite powder.
2. Description of the Prior Arts:
Various methods have been known as a development system in an electrophotography. Two component system for transferring a toner made of a mixture of carbon and a resinous component through a magnetic brush made of an iron powder carrier on an electrophotosensitive substrate has been mainly employed.
Thus, one component system for using a toner having magnetic property which contains a magnetic powder instead of carbon without a carrier has been developed and employed in a commercial scale.
In the one component system, an operation of a development is easily carried out and accordingly, no control is required and an exchange of a carrier is not required and only additional feeding of the toner is required. Moreover, a development unit is simple whereby a labour required for a maintenance is highly reduced and an apparatus is simplified to result light weight and low cost.
In usual, the magnetic powder for the magnetic toner used in the one component system should have the following characteristics.
(i) It is necessary to have a high magnetic flux density in a magnetic field of about 1000 Oe. For example, in an external magnetic field of 1000 Oe, it is necessary to have a maximum magnetizing force that is, magnetization, .sigma.m of higher than 40 emu/g. so that a height of the magnetic brush is enough high.
(ii) It is necessary to have a high coercive force together with the requirement (i). For example, in an external magnetic field of 1000 Oe, it is necessary to have a coercive force Hc of about 150 to 500 Oe so that desired characteristics for transfer property, fluidity and coercive force of the toner are given. Thus, it is necessary to have .sigma.m.times.Hc of more than about 6.times.10.sup.3 as B-H product.
(iii) It is necessary to have suitable electric resistivity preferably 10.sup.2 to 10.sup.7 .OMEGA..cm as an electric resistivity of the magnetic powder.
(iv) It is necessary to have black color which can be practical used. A coloring agent can be incorporated in the magnetic toner. However, it is preferable not to incorporate a coloring agent by using a magnetic powder having black color.
(v) It is necessary to have high heat resistance and to have stable hue especially black color and stable electromagnetic characteristics in a range of about 0.degree. to 150.degree. C.
(vi) It is necessary to have less hygroscopic property and high moisture resistance since electrostatic characteristics of the toner are disadvantageously varied when the hygroscopic property is high.
(vii) It is necessary to have high compatibility of the magnetic powder to the resin. In usual, the toner has a diameter of less than several tens .mu. and accordingly, micro-compatibility in the toner is important factor. Thus, it is necessary to use the magnetic powder having a diameter of less than 1.mu. and a sharp particle size distribution and uniform particle size among many lots in its production.
(viii) It is necessary to prevent serious deterioration of electrostatic characteristics of the resin, deterioration of the resin and periodical change of the properties.
On the other hand, it has been proposed to use ferromagnetic alloys such as magnetites and ferrites and alloys which have not ferromagnetic property but impart ferromagnetic property by a heat-treatment such as Mn-Cu-Al alloy and chromium dioxide etc. as a magnetic powder for magnetic toner in Japanese Unexamined Patent Publication No. 45639/1975.
Thus, the magnetic powder should be pulverized in a form of fine powder when it is used for a magnetic toner. These alloys are unstable in the pulverization and the cost thereof is expensive. On the other hand, chromium dioxide has toxicity. Thus they may not be practically used. It has been proposed to use ferrite in a toner in patents and other prior arts. However, these proposals are only suggestions. Any ferrite having specific components and formula has not been practically used for a magnetic toner.
It has been proposed to use in a toner, a magnetite as iron black used for a pigment which is obtained as a precipitate in a reaction of an aqueous solution (hereinafter referring to as magnetite obtained by an aqueous solution process) in patents and other prior arts. The magnetite has been practically used. The magnetite has satisfactory electric and magnetic characteristics required in the items (i) to (iii) and satisfactory hue in the item (iv). However, it is difficult to control the electric and magnetic characteristics required in the items (i) to (iii) in satisfactory uniform accuracy in the production of the magnetite. These characteristics may be varied in each batch in the production. The heat resistance, the moisture resistance and the compatibility to the resin and the adverse effect to the resin required in the items (v) to (viii) are not satisfactory and may be varied in each batch in the production. These disadvantages are found. It is difficult to satisfy these requirements of the characteristics by the magnetite obtained by the aqueous solution process, because there are many fluctural factors for each lot so as to highly vary the electric and magnetic characteristics, the heat resistance, the moisture resistance, the particle diameter, the particle size distribution and the impurity content.
When the magnetite is used for the toner, various problems are caused in the use of the toner and a trouble is caused in a copying process because of said disadvantages.
In the production of the magnetite obtained by the aqueous solution process, a large amount of a base is used whereby a washing is not easy and a labour is required for a treatment of a waste solution after the washing to increase a cost for the production. These are disadvantages in the production.
It has been studied on maghemite producted by a similar process to that of the magnetite or modified maghemite obtained by incorporating cobalt so as to vary the magnetic characteristics. Thus, these ones have also similar disadvantages to those of the magnetite.